A tree disease causing dark discoloration inside the wood, or used metaphorically to describe someone who is fundamentally evil or wicked.
Compound of 'black' + 'heart,' literally describing the dark interior of diseased wood. By extension, it became a metaphor for moral corruption or evil nature.
The literal botanical meaning of blackheart—a fungal disease that blackens a tree's interior while leaving the exterior looking healthy—became the perfect metaphor for hidden evil, which is why Shakespeare and other writers loved using 'black heart' symbolically.
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